It has been argued that current saturation in graphene field-effect transistors (GFETs) is needed to get optimal maximum oscillation frequency (f_max). This paper investigates whether velocity saturation can help to get better current saturation and if that correlates with enhanced f_max. We have fabricated 500 nm GFETs with high extrinsic f_max (37 GHz), and later simulated with a drift–diffusion model augmented with the relevant factors that influence carrier velocity, namely: short-channel electrostatics, saturation velocity effect, graphene/dielectric interface traps, and self-heating effects. Crucially, the model provides microscopic details of channel parameters such as carrier concentration, drift and saturation velocities, allowing us to correlate the observed macroscopic behavior with the local magnitudes. When biasing the GFET so all carriers in the channel are of the same sign resulting in highly concentrated unipolar channel, we find that the larger the drain bias is, both closer the carrier velocity to its saturation value and the higher the f_max are. However, the highest f_max can be achieved at biases where there exists a depletion of carriers near source or drain. In such a situation, the highest f_max is not found in the velocity saturation regime, but where carrier velocity is far below its saturated value and the contribution of the diffusion mechanism to the current is comparable to the drift mechanism. The position and magnitude of the highest f_max depend on the carrier concentration and total velocity, which are interdependent and are also affected by the self-heating. Importantly, this effect was found to severely limit radio-frequency performance, reducing the highest f_max from ∼60 to ∼40 GHz.

中文翻译：

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载流子速度饱和是否有助于提高石墨烯场效应晶体管的 fmax？

有人认为，石墨烯场效应晶体管 (GFET) 中的电流饱和是获得最佳最大振荡频率 ( f _max ) 所必需的。本文研究了速度饱和是否有助于获得更好的电流饱和，以及这是否与增强的f _max相关。我们制造了具有高外在f _{max的 500 nm GFET}（37 GHz），然后用漂移扩散模型进行模拟，该模型增加了影响载流子速度的相关因素，即：短通道静电、饱和速度效应、石墨烯/电介质界面陷阱和自热效应。至关重要的是，该模型提供了诸如载流子浓度、漂移和饱和速度等通道参数的微观细节，使我们能够将观察到的宏观行为与局部幅度相关联。当对 GFET 进行偏置以使沟道中的所有载流子具有相同的符号导致高度集中的单极沟道时，我们发现漏极偏置越大，载流子速度越接近其饱和值，f _max越高。然而，最高f _max可以在源极或漏极附近存在载流子耗尽的偏置下实现。在这种情况下，在速度饱和状态下没有发现最高f _max，但在载流子速度远低于其饱和值的情况下，扩散机制对电流的贡献与漂移机制相当。最高f _max的位置和大小取决于载流子浓度和总速度，它们是相互依赖的，也受自热的影响。重要的是，发现这种效应严重限制了射频性能，将最高f _max从 ~60 GHz 降低到 ~40 GHz。